Bagworms (Thyridopteryx ephemeraeformis)

Bagworms (Thyridopteryx ephemeraeformis) are destructive insects that attack many deciduous and evergreen trees and shrubs. Young bagworm caterpillars feed on needles and leaves and a large infestation can lead to almost complete defoliation of trees. Some of the more common evergreen host plants include arborvitae (Thuja), fir (Abies), hemlock (Tsuga), juniper (Juniperus), pine (Pinus) and spruce (Picea). Deciduous host plants include black locust (Robinia pseudoacacia), honeylocust (Gleditsia triacanthos), sweetgum (Liquidambar styraciflua) and sycamore (Platanus occidentalis).

Life Cycle:

Bagworms can be recognized by the characteristic case or "bag" that they construct around themselves. The bag is made from silk and plant material from the plants they are feeding on.

Eggs of bagworms hatch in late May and early June from bags that were constructed the previous season. The larvae (caterpillars) construct a bag around their hind parts after hatching and carry it around as they feed on needles and leaves throughout the summer.

In August, the mature larvae attach their bags to a branch (looking a lot like small pine cones) where they stay for about 4 weeks in a pupa stage.

Males emerge from the cases in September or October in search of females. The adult females remain in their bag, releasing a pheromone to attract the males. They mate and lay their eggs without ever leaving the bag.

Female bagworms can lay up to 1,000 eggs which overwinter in the bags and hatch out the following May.

Here are some tips on getting rid of bagworms:

As much as possible, try to physically remove and destroy as many of the bags as you can. Depending on the size of the trees and shrubs, this may or may not be feasible. We do not recommend using ladders!

The second most important step is to control the YOUNG caterpillars when they hatch out in the later part of May. Young caterpillars can be controlled safely (without harming beneficial insects) by spraying Bonide BT Thuricide (Bacillus thuringiensis). This bacteria infects and kills the young caterpillars. It is also helpful for controlling young tent caterpillars. A dust formulation of BT is sold as Dipel. This also works but the liquid formulation is probably easier to use for trees and shrubs. Always read and follow the label directions when using any pesticide.

If you miss the younger ones, larger caterpillars can be controlled using a broad-spectrum insecticide in the later part of June according to label directions. NEVER spray any insecticide when trees are in bloom!

The best and safest/environmentally friendly way to deal with these pests is removal of the bags and/or trying to kill the young caterpillars using BT.